1. Field of the Invention
The present invention relates to a content addressable memory (CAM) to be used to convert a virtual address to a physical address in a microprocessor.
2. Related Art
In microprocessors, a content addressable memory is typically used to convert a virtual address to a physical address (see Japanese Patent Application Laid-Open Publication Nos. 2000-235787 and 2002-163891).
FIG. 8 is a circuit diagram showing an internal configuration of a conventional content addressable memory (CAM). The content addressable memory includes as many comparison circuits 31 as the number of bits provided for each of a plurality of entries, and precharge circuits 32 each connected to an output line L1 for comparison circuits 31 of the same entry. Each comparison circuit 31 includes an SRAM cell 33 which stores a reference bit sequence, transistors Q21 and Q22 which controls reading/writing data from/into the SRAM cell 33, a comparator 34 which compares data stored in the SRAM cell 33 with comparison data inputted from the outside, and a transistor Q23, which is turned on and off according to an output of the comparator 34.
The precharge circuit 32 is a so-called wired-NOR including a transistor Q24 which precharges the output line L1 to a high level, a transistor Q25 and an inverter IV 11 which holds a logic of the output line L1.
The output line L1 is precharged to the high level by the precharge circuit 32. If noncoincidence is detected in any one of the comparison circuits 31, then the output line L1 becomes a low level.
The content addressable memory shown in FIG. 8 includes as many such wired-NORs as the number of entries. When comparison circuits 31 conduct comparison operation, all wired-NORs operate because of the characteristics, and noncoincidence occurs in almost all wired-NORs, resulting in a problem of very large current consumption.
In a multi-comparison content addressable memory shown in FIG. 9 obtained by expanding the content addressable memory shown in FIG. 8, the number of wired-NORs increases and the problem of the current consumption is further aggravated. Even in a time-divisional multi-comparison CAM which performs multi-comparison in time division in order to restrain increase of layout, there is the problem in which current consumption increases.